Conventional vacuum cleaners typically have a primary dust and dirt separating unit or dirt collector, such as a primary filter bag, primary filter element, or other dirt separation device such as a cyclonic separation device, into which the dirt-laden incoming vacuum airflow is directed from the vacuum hose, for example. After most of the dust and dirt are separated from the airflow in the primary dirt collector, the primary filtered airflow then optionally passes through a motor protection filter to the suction motor unit including the vacuum blower or fan and the drive motor. From there, the airflow is blown by the motor unit into an outlet or exhaust channel that leads to a plurality of adjacently arranged outlet slots or exhaust openings in the vacuum cleaner housing, through which the airflow is exhausted, i.e. blown out from the vacuum cleaner to the surrounding atmosphere.
In typical conventional vacuum cleaners, the exhausted airflow is emitted from the exhaust openings of the housing with a substantially high flow velocity. Also, the exhaust airflow is often emitted in a relatively concentrated jet stream. Due to this relatively strong localized concentration of the exhaust airflow, and the high flow velocities thereof, flow-generated noise is emitted, which represents a substantial portion of the total noise emission of the operating vacuum cleaner. Furthermore, a portion of the noise emission of the motor unit is also transmitted to and emitted from the exhaust openings of the housing. In addition to the emitted noise, the exhaust airflow in the manner of a concentrated jet has other undesirable influences, such as the strong blowing effect that is very noticeable by the person using the vacuum cleaner.
For flowtechnical reasons, e.g. to increase the operating efficiency of the vacuum cleaner by minimizing the air flow resistance, vacuum cleaner designs strive to provide the largest possible cross-sectional area of the exhaust channel and especially the exhaust outlet openings in the housing of the vacuum cleaner, in order thereby to achieve a small exhaust air flow resistance. However, contrary to these requirements, it is also desired to reduce the noise emissions as discussed above, which could be achieved by providing a relatively large air flow resistance. Thus, these two requirements, of efficient operation through a reduced flow resistance on the one hand, and reduced noise emissions through an increased flow resistance on the other hand, are directly opposed to each other.
European Patent Publication EP 0,706,774 B discloses an attempt to improve on the above mentioned balancing between the two opposite goals. Particularly, this European Patent Publication suggests to increase the outlet cros-sectional area through corresponding covered openings in the form of slots in the side walls of the housing of the vacuum cleaner in the manner of a protective shroud.